The present invention relates to a new and improved method of, and apparatus for, detecting at least one reducing gas in a gas mixture to be investigated, particularly in air.
In its more particular aspects, the present invention relates specifically to a new and improved method of detecting at least one reducing gas in a gas mixture to be investigated, particularly in air, by means of a gas detector which contains a gas sensor comprising a metal oxide semiconductor. A selected property, such as the electrical conductivity of the gas sensor depends directly upon the concentration of the at least one reducing gas. The temperature of the gas sensor is continuously increased according to a predetermined pattern from a starting value to an upper threshold value and is subsequently decreased again to the starting value according to the same or a different pattern. The gas sensor is subjected to at least two such heating cycles. An electrical output signal of the gas sensor is evaluated in an evaluation circuit arrangement.
In such a method as known, for example, from European Patent Publication No. 0,092,068, published Oct. 26, 1983, a sensor element which is responsive to gases, is subjected to at least two temperature cycles according to a predetermined pattern which is optimized for predetermined gases. In each such temperature or heating cycle the sensor element is continuously heated from a starting value to an upper value and subsequently the temperature is lowered again to the starting value according to the same or according to a different pattern. An evaluation circuit arrangement compares the signal supplied by the sensor element during the temperature or heating cycles. The signal delivered by the sensor element is dependent upon the composition of the gas or vapor atmosphere and the delivered signal is compared with stored values which are characteristic for the presence of predetermined gas and/or vapor components.
By using such method it is possible to fully utilize the broad-band sensitivity range of gas sensors based on metal oxides and to simultaneously and selectively detect individual gases. During the use of such metal oxide semiconductor gas sensors in gas monitoring installations it is required to detect toxic or explosive gases in concentration ranges which are as far as possible below the lower explosion limit. Therefore, there is still a need for methods by means of which trace quantities of explosive gases can be detected.
In a further method as known, for example, from German Patent Publication No. 2,313,413, published Sept. 26, 1974, the carbon monoxide and/or the methane content of a gas mixture present in underground operations is determined by means of measuring the electrical resistance of a metal oxide semiconductor which adsorbs the gas component to be measured and desorbs such component in an accelerated manner at higher temperatures. Prior to each measurement the temperature of the metal oxide semiconductor is varied from a lower threshold value to a higher threshold value and the measurement is conducted at the lower threshold value. The variation of the electrical resistance is measured through a predetermined time interval and the concentration of carbon monoxide and/or methane is determined from the absolute measured value and/or from the variation in time of the measuring signal.
The method described in the aforementioned '413 German patent publication is adapted to the requirements of the mining industry. It has the advantage that a good long-term stability is achieved by periodically heating the metal oxide semiconductor. For detecting carbon monoxide there is required a second measuring chamber which contains a continuously heated metal oxide semiconductor in order to compensate for the interfering sensitivity towards methane-type components. The regeneration of the metal oxide semiconductor between the individual measuring periods requires about 1 to 5 minutes. Subsequently, the metal oxide semiconductor is required to be cooled to room temperature before the actual measuring operation can be started. Such method is therefor unsuited for use in alarm installations in which a rapid recognition of a dangerous concentration of combustible gases is required. Furthermore, the known method is not particularly sensitive because the absolute measured value or the variation of the measuring signal with time is utilized for determining the gas concentration.
A carbon monoxide detector as known, for example, from German Patent Publication No. 2,832,828, contains two carbon monoxide detectors of different sensitivities for reducing the influence of other gases which are simultaneously present in a gas sample. The different sensitivities against carbon monoxide are obtained by different structures of the metal oxide semiconductors which are contained in the carbon monoxide detectors or by different temperatures of the detector elements. Such principle is ineffective for other gases.
In the Japanese Patent Publication No. Sho 49-11997 there is described a gas and smoke detector which comprises two measuring chambers which contain metal oxide semiconductor sensors and which are differently accessible for the air to be investigated in order to suppress the sensitivity towards slow changes in the air properties like, for example, temperature and humidity. The difference of the measuring signals obtained from the two sensors is utilized for evaluation. Due to the ready diffusion, particularly of low-molecular gases the difference between the two measuring signals soon again becomes zero, so that such detector would be rather suited for smoke detection.
In a method for detecting combustible gases as known, for example, from German Pat. No. 1,017,384, granted Oct. 10, 1957, the detection is based on a catalytic combustion of the gases to be detected at a platinum filament and the variation of the resistance thereof is used as the measured magnitude. In this process the platinum filament alternatingly serves as the combustion mass and as a comparison mass by alternatingly passing the gas mixture to be investigated over the platinum filament and a reference gas which contains none or only a small content of the combustible components. The temperature of the platinum filament varies in time between a maximum value corresponding to the combustion period and a minimum value corresponding to a reference gas which contains no combustible components. The evaluation of such temperature variations or fluctuations results in an a.c.-voltage signal, the frequency of which is dependent upon the gas exchange period and the amplitude of which is dependent upon the content of combustible components. The amplitude is then utilized for determining the content of combustible gases.
Using the aforementioned method there can be avoided one disadvantage of the gas detectors operating with the use of catalytic combustion, namely the zero drift which is due to the physical changes in the catalyst. Other disadvantages of such detectors are, for example, their sensitivity towards catalyst poisoning by interfering gases and their relative insensitivity, and these disadvantages could not be eliminated by using the method proposed in the aforementioned German Pat. No. 1,017,384. Specifically, one disadvantage of the gas sensors which operate according to the principle of catalytic combustion could not be redressed: the temperature of the platinum filament is determined by the heat of combustion of the combustible gases to be detected and the electrical resistance of the platinum wire is dependent upon the temperature. The temperature of the platinum filament, however, is further dependent on a number of other conditions so that the obtained results can not be unequivocally interpreted.
Since the obtained electrical signals are dependent upon the temperature of the platinum filament, they are also dependent on the heat conductivity of the measuring or combustion chamber which is determined by the geometry of the measuring chamber. Already small and accidental deviations in the size and thickness of the platinum filament and in its position relative to the surrounding space and the walls thereof can result in differences of signal levels up to a factor of 10. Also, there can not be eliminated detrimental effects which are due to different flow conditions and accidental depositions close to the platinum filament which also are accompanied by a change in the thermal conductivity.